如何在Java中正确使用try-catch-finally块

Question

使用finally块关闭我所在的文件的正确方法是什么:event.dat。如果你能帮我弄明白这件事，我将不胜感激。我真的花了4个小时移动东西，玩代码，在网上搜索答案，但无济于事。没有这部分代码工作得很好，但我需要知道它如何为未来的实例工作。谢谢你，祝你有美好的一天!

我对这个块有问题:

          finally{
              fstream.close();        
          } 

位于以下代码中:

    String answer;
    String letter;
    String inType = "";
    double inAmount = 0;
    double amount;
    description();
    GironEventClass newInput = new GironEventClass();
    try{
        File infile = new File("event.dat");
        Scanner fstream = new Scanner(infile);
        System.out.println("File Contents ");
        while(fstream.hasNext())
        {
            inAmount = fstream.nextInt();
            inType = fstream.next();
            try{
                newInput.donations(inType, inAmount);
            }
            catch(IllegalArgumentException a){
                System.out.println("Just caught an illegal argument exception. ");
            }  
           finally{
                   fstream.close();        
          }  
        }
        System.out.println("Total Sales: " + newInput.getSale());
        System.out.println("Donations: " + newInput.getDonated());
        System.out.println("Expenses: " + newInput.getExpenses());

    }
    catch (FileNotFoundException e){
        System.out.println("nEvent.dat could not be opened. ");
    }
    do{
        System.out.print("Are there any more items to add that were not in the text file? (Type 'Y' or 'N')");
        answer = keyboard.next();
        if (("Y".equals(answer)) || ("y".equals(answer)))
        {
            letter = inLetter();
            amount = inAmount();
            newInput.donations(letter, amount);
        }
    }while (("Y".equals(answer)) || ("y".equals(answer)));
    newInput.display();
}
public static String inLetter(){
    Scanner keyboard = new Scanner(System.in);
    String result;
    String resultTwo;
    System.out.println("T = Tiket Sales");
    System.out.println("D = Donations");
    System.out.println("E = Expenses");
    System.out.print("Please input an identifier ");
    result = keyboard.nextLine();
    resultTwo = result.toUpperCase();
    return resultTwo;    
}
public static double inAmount(){
    Scanner keyboard = new Scanner(System.in);
    double result;
    System.out.println("Please input an amount ");
    result = keyboard.nextInt();
    if(result <= 0.0){
        System.out.print("Please input a positive and non-zero amount ");
        result = keyboard.nextInt();
    }
    return result;
}
public static void description(){
    System.out.println("The program will ask you what amount is being spent on what.");
    System.out.println("    ex: expenses, ticket sales, and profts.");
    System.out.println("This program will help determine whether the event generated or lost money.");      
}

Answer 1

扫描仪应该是这样工作的:

while scanner has object
   read them ( one object per method's call) 
when objects are done
   close the reader.

您的问题是当while结论为真时使用close函数。所以你应该把它放在while循环之外

Answer 2

Try块尝试做某事。

Catch块只在Try块出错时执行。

Finally块每次都在Try块(和Catch块，如果执行的话)之后执行。

你的问题是你试图在while循环中关闭fstream。

while(fstream.hasNext())  <----- referencing fstream
        {
            inAmount = fstream.nextInt();
            inType = fstream.next();
            try{
                newInput.donations(inType, inAmount);
            }
            catch(IllegalArgumentException a){
                System.out.println("Just caught an illegal argument exception. ");
            }  
           finally{
                   fstream.close();        <---- closing said stream
          }  
        }

既然你已经关闭了流，这个while循环应该只执行一次。也就是说，在while循环之外关闭fstream，程序应该恢复正常。

还可以将while循环移动到try块中，这也可以。

Answer 3

你需要在try块之外声明fstream，否则它将在finally块中不可见。

File infile = null;
File infile = null;
try {
    infile = new File("event.dat");
    fstream = new Scanner(infile);
    ...
} catch ( ) {
    ...
} finally {
   // You need to handle exceptions also here
   infile.close();
   fstream.close();
}

你也可以使用新的try with resources语法，把流的闭包留给jvm。